Polymeric liquid metal optical switch

ABSTRACT

In accordance with the invention, a piezoelectrically actuated relay that switches and latches by means of a liquid metal is disclosed. The relay operates by means of a plurality of shear mode piezoelectric elements used to cause a pressure differential in a pair of fluid chambers. Differential pressure is created in the chambers by contracting and expanding the chambers due to action by the piezoelectric elements. The differential pressure causes the liquid metal drop to overcome the surface tension forces that would hold the bulk of the liquid metal drop in contact with the contact pad or pads near the actuating piezoelectric element. The switch latches by means of surface tension and the liquid metal wetting to the contact pads.

BACKGROUND

[0001] Piezoelectric materials and magnetostrictive materials(collectively referred to below as “piezoelectric materials”) deformwhen an electric field or magnetic field is applied. Thus piezoelectricmaterials, when used as an actuator, are capable of controlling therelative position of two surfaces.

[0002] Piezoelectricity is the general term to describe the propertyexhibited by certain crystals of becoming electrically polarized whenstress is applied to them. Quartz is a good example of a piezoelectriccrystal. If stress is applied to such a crystal, it will develop anelectric moment proportional to the applied stress.

[0003] This is the direct piezoelectric effect. Conversely, if it isplaced in an electric field, a piezoelectric crystal changes its shapeslightly. This is the inverse piezoelectric effect.

[0004] One of the most used piezoelectric materials is theaforementioned quartz. Piezoelectricity is also exhibited byferroelectric crystals, e.g. tourmaline and Rochelle salt. These alreadyhave a spontaneous polarization, and the piezoelectric effect shows upin them as a change in this polarization. Other piezoelectric materialsinclude certain ceramic materials and certain polymer materials. Sincethey are capable of controlling the relative position of two surfaces,piezoelectric materials have been used in the past as valve actuatorsand positional controls for microscopes. Piezoelectric materials,especially those of the ceramic type, are capable of generating a largeamount of force. However, they are only capable of generating a smalldisplacement when a large voltage is applied. In the case ofpiezoelectric ceramics, this displacement can be a maximum of 0.1% ofthe length of the material. Thus, piezoelectric materials have been usedas valve actuators and positional controls for applications requiringsmall displacements.

[0005] Two methods of generating more displacement per unit of appliedvoltage include bimorph assemblies and stack assemblies. Bimorphassemblies have two piezoelectric ceramic materials bonded together andconstrained by a rim at their edges, such that when a voltage isapplied, one of the piezoelectric materials expands. The resultingstress causes the materials to form a dome. The displacement at thecenter of the dome is larger than the shrinkage or-expansion of theindividual materials. However, constraining the rim of the bimorphassembly decreases the amount of available displacement. Moreover, theforce generated by a bimorph assembly is significantly lower than theforce that is generated by the shrinkage or expansion of the individualmaterials.

[0006] Stack assemblies contain multiple layers of piezoelectricmaterials interlaced with electrodes that are connected together. Avoltage across the electrodes causes the stack to expand or contract.The displacements of the stack are equal to the sum of the displacementsof the individual materials. Thus, to achieve reasonable displacementdistances, a very high voltage or many layers are required. However,conventional stack actuators lose positional control due to the thermalexpansion of the piezoelectric material and the material(s) on which thestack is mounted.

[0007] Due to the high strength, or stiffness, of piezoelectricmaterial, it is capable of opening and closing against high forces, suchas the force generated by a high pressure acting on a large surfacearea. Thus, the high strength of the piezoelectric material allows forthe use of a large valve opening, which reduces the displacement oractuation necessary to open or close the valve.

[0008] With a conventional piezoelectrically actuated relay, the relayis “closed” by moving a mechanical part so that two electrode componentscome into electrical contact. The relay is “opened” by moving themechanical part so that the electrode components are no longer inelectrical contact. The electrical switching point corresponds to thecontact between the electrode components of the solid electrodes.

[0009] Liquid metal micro switches have been developed that use liquidmetal as the switching element and the expansion of a gas when heated toactuate the switching function. The liquid metal has some advantagesover other micromachined technologies, such as the ability to switchrelatively high power approximately 100 mW) using metal-to-metalcontacts without microwelding, the ability to carry this much powerwithout overheating the switch mechanism and adversely affecting it, andthe ability to latch the switching function. However, the use of aheated gas to actuate the switch has several disadvantages. It requiresa relatively large amount of power to change the state of the switch,the heat generated by switching must be rejected effectively if theswitch duty cycle is high, and the actuation speed is relatively slow,i.e., the maximum switching frequency is limited to several hundredHertz.

SUMMARY

[0010] The present invention uses a piezoelectric method to actuateliquid metal switches. The actuator of the invention uses piezoelectricelements in a sheart mode rather than in a bending mode. A piezoelectricdriver in accordance with the invention is a capacitive device whichstores energy rather than dissipating energy. As a result, powerconsumption is much lower, although the required voltages to drive itmay be higher. Piezoelectric pumps may be used to pull as well as push,so there is a double-acting effect not available with an actuator thatis driven solely by the pushing effect of expanding gas. Reducedswitching time results from use of piezoelectric switches in accordancewith the invention.

[0011] A piezoelectrically actuated liquid metal switch in accordancewith the invention is comprised of a plurality of layers. Liquid metalis contained within a channel in one layer and contacts switch pads on acircuit substrate. The amount and location of the liquid metal in thechannel is such that only two pads are connected at a time. The metal ismovable so that it contacts the center pad and either end pad bycreating an increase in pressure between the center pad and the firstend pad such that the liquid metal breaks and part of it moves toconnect to the other end pad. A stable configuration results due to thelatching effect of the liquid metal as it wets to the pads and is heldin place by surface tension.

[0012] An inert and electrically nonconductive liquid fills theremaining space in the switch. The pressure increase described above isgenerated by the motion of a piezoelectric pump or pumps. The type ofpump of the invention utilized the shearing action of piezoelectricelements in a pumping cavity to create positive and negative volumechanges. These actions may cause pressure decreases, as well asincreases, to assist in moving the liquid metal.

DESCRIPTION OF THE DRAWINGS

[0013] The invention can be better understood with reference to thefollowing drawings. The components in the drawings are not necessarilyto scale, emphasis instead being placed upon clearly illustrating theprinciples of the present invention.

[0014]FIG. 1 shows a side view of the layers of a piezoelectric metalswitch in accordance with the invention.

[0015]FIG. 2 shows a side cross section of a side view of the layers ofa piezoelectric switch in accordance with the invention.

[0016]FIG. 3A shows a top level view of the orifice layer.

[0017]FIG. 3B is a side-sectional view of the orifice layer.

[0018]FIG. 4 shows a top level view of the substrate layer with theswitch contacts.

[0019]FIG. 5A is a top view of the liquid metal channel layer.

[0020]FIG. 5B is a side-sectional view of the liquid metal channellayer.

[0021]FIG. 6 is a top view of the piezoelectric layer showing two setsof piezoelectric elements.

[0022]FIG. 7 is a top view of the piezoelectric layer showing the“switch actuator cavity” expanded for the right hand set ofpiezoelectric elements.

[0023]FIG. 8 is a top view of the piezoelectric layer showing the“switch actuator cavity” contracted for the right hand set ofpiezoelectric elements.

[0024]FIG. 9A shows a top view of the actuator fluid reservoir layer.

[0025]FIG. 9B shows a side-sectional view of the actuator fluidreservoir layer.

[0026]FIG. 10 shows an alternate side cross section of a side view ofthe layers of a piezoelectric switch in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027]FIG. 1 is a side view of an embodiment of the invention showingfive layers of a relay 100. The top layer 110 is an actuator fluidreservoir layer and acts as a reservoir for fluid used in the actuator.The second layer 120 is an orifice layer. The orifice layer is optionaland provides orifices for between the top layer 110 and the layersbelow. The third layer 130 is a piezoelectric layer which houses apiezoelectric switching mechanism. The fourth layer 140 is a liquidmetal channel layer and houses a liquid metal used in the switchingmechanism. The substrate layer 150 acts as a base and provides a commonfoundation for a plurality of circuit elements that may be present.

[0028]FIG. 2 shows a cross sectional view of an embodiment of anactuator 100 in accordance with the invention. FIG. 2 is a crosssectional view of FIG. 1. The actuator fluid reservoir layer 110 has achamber 150 that contains a volume of actuator fluid. The actuator fluidis an inert, electrically non-conductive fluid. This fluid is preferablya low viscosity inert organic liquid such as a low molecular weightperfluorocarbon such as is found in the 3M line of Fluorinert products.It may alternatively consist of a light mineral or synthetic oil, forexample. The orifice layer 120 is adjacent to the reservoir layer 110.Two openings 160 in the orifice layer 120 coincide with openings in thereservoir 150. The orifice layer 120 is optional and provides a boundarylayer between the reservoir layer 110 and the piezoelectric layer 130.

[0029] The piezoelectric layer 130 houses a plurality of piezoelectricelements 170 utiized in the relay 100. Each of the of piezoelectricelements 170 in FIG. 2 is paired with another of the piezoelectricelements 170 which form sets of pairs of piezoelectric elements 170 Eachpair of piezoelectric elements 170 form a chamber 175. Each chamber 175coincide with the orifices 160 so that fluid can flow from the reservoir150 into and out of the chamber 175. The piezoelectric layer 130 hasopenings 180 that coincide with the chambers 175 opposite the orifices160.

[0030] The liquid metal layer 140 comprises a liquid metal 190 which iscontained within a channel 195 and a set of switch contact pads 200located on the circuit substrate 150. The space in the channel 195 whichis not filled with liquid metal 190 is filled with the fluid. The liquidmetal is inert and electrically conductive. The amount and location ofthe liquid metal 190 is such that only two pads 200 are connected at atime. The center pad 200 will always be contacted and either the left orright pad 200. In the embodiment of the invention shown in FIG. 2, theliquid metal 190 is in contact with the center pad 200 and the right pad200. The liquid metal 190 is moved to contact the left pad 200 by theaction of the piezoelectric elements 160 which causes pressuredifferentials in chambers 175.

[0031] Bending of the piezoelectric elements 170 causes either anincrease or a decrease in chamber 175. An increase in pressure inchamber 175 causes the liquid metal 190 to move leftward until it iscontacting the center pad 200 and the left pad 200. The pumping actionsof the piezoelectric elements create either a positive or a negativevolume, and pressure, change in chambers 175. When the right set ofpiezoelectric elements 170 causes an increase in pressure—decreasedvolume—the left side can cause a decrease in pressure—increased volume.The opposite movements of the two sets of piezoelectric elements 160assist in movement of the liquid metal 200.

[0032] In a preferred embodiment of the invention, the liquid metal 190is mercury. In an alternate preferred version of the invention, theliquid metal is an alloy containing gallium.

[0033] In operation, the switching mechanism of the invention operatesby shear mode displacement of the piezoelectric elements 170. Anelectric charge is applied to the piezoelectric elements 170 whichcauses the elements 170 to bend by shear mode displacement. Each set ofpiezoelectric element 170 work together. As discussed above, the bendingaction of the piezoelectric elements 170 can be on an individual basis,i.e. each set separately—or in a cooperative manner—both sets together.Inward bending of the piezoelectric elements 160 of one of the setscauses an increase of pressure and decrease of volume in the chamber 180directly below the outward bending set. This change in pressure/volumecauses displacement of the moveable liquid metal 190. To increase theeffectiveness, the piezoelectric elements of the other set can bendinward at the same time. Reversing the bending motion of thepiezoelectric elements 160 causes the liquid metal 190 to displace inthe opposite direction. The piezoelectric elements 160 are relaxed, i.e.the electric charge is removed, once the liquid metal 190 has displaced.The liquid metal 190 wets to the contact pads 200 causing a latchingeffect. When the electric charge is removed from the piezoelectricelements 160, the liquid does not return to its original position butremains wetted to the contact pad 200.

[0034]FIG. 3A is a top view of the orifice layer 120. The two orifices160 provide flow restriction for the fluid between the reservoir 150 andthe chambers 175 in the piezoelectric layer 130. FIG. 3B is a sidesectional view at A-A of the orifice layer 120. The orifices 175 areshown extending through the layer 120.

[0035]FIG. 4 shows a top level view of the substrate layer 150 with theswitch contacts 200. The switch contacts 200 can be connected throughthe substrate 150 to solder balls (not shown) on the opposite side forthe routing of signals. It is understood that there are alternatives torouting of signals. For instances, the signal routing can be placed inthe substrate layer 150. It is also understood that the switch pads 200in FIG. 2 are merely representative of the switch pads of the invention.Specifically, the substrate layer 150 and the switch pads 200 are notnecessarily proportional to the switch pads and substrate layer in FIG.4.

[0036]FIG. 5A is a top view of the liquid metal channel layer 130. Theliquid metal layer 140 comprises the liquid metal channel 195 and a pairof through-holes 180 which act as the conduits for movement of liquidfrom the liquid metal channel 195 and the chamber 175 shown in FIG. 2.FIG. 4B is a side-sectional view of the liquid metal layer 140 at theA-A point. The liquid metal channel 195 is shown connecting to thethrough-hole 180.

[0037]FIG. 6 is a top view of the piezoelectric layer 120 showing twosets of piezoelectric elements 170. Each pair of piezoelectric elements170 form a chamber 175. Each chamber 175 coincides with the orifices 160(not shown) so that fluid can flow from the reservoir 150 (not shown)into and out of the chamber 175.

[0038]FIG. 7 shows a top view of the piezoelectric layer 120 showing twosets of piezoelectric elements 170. The pair of piezoelectric elements170 on the right side of the figure have been activated to bend(deflect) outward. The deflected piezoelectric elements 170 form anexpanded pumping cavity 210. The expanded pumping cavity 210 pulls fluidfrom the liquid metal channel 195 (not shown) causing liquid metal 190(not shown) to be pulled toward the right side.

[0039]FIG. 8 shows a top view of the piezoelectric layer 120 showing twosets of piezoelectric elements 170. The pair of piezoelectric elements170 on the right side of the figure have been activated to bend(deflect) inward. The deflected piezoelectric elements 170 form acontracted pumping cavity 220. The contracted pumping cavity 220 pushesfluid from the liquid metal channel 195 (not shown) causing liquid metal190 (not shown) to be pushed toward the left side.

[0040] It is understood that the sets of piezoelectric elements 170 canwork cooperatively. For instance, when one set of elements 170 deflectsoutward as shown in FIG. 7, the other set of elements 170 can deflectinward as shown in FIG. 8. Cooperative action increases the actionproduced on the fluid increasing the forces causing the liquid metal tomove.

[0041]FIG. 9 shows a top view of the actuator fluid reservoir layer 110with the reservoir 150 and a fill port 230. The fluid reservoir 150 isillustrated here as a single part in one embodiment of the invention. Inan alternate embodiment of the invention, the fluid reservoir is madefrom multiple sections. The fluid reservoir 150 is a depository of theworking fluid and has a compliant wall to keep pressure pulseinteractions between pumping elements—crosstalk—to a minimum. The fluidreservoir 150 is filled after the switch assembly 100 has beenassembled. The fill port 230 is sealed after the reservoir has beenfilled.

[0042]FIG. 10 shows an alternate embodiment of the invention wherein thefluid reservoir comprises multiple compartments 240. The wall 250separating the multiple compartments has a pressure relief port 260which connects to both of the compartments 240 which equalizes thepressure between compartments 240, and each of the compartments 240 hasa compliant exterior wall which keeps pressure pulse interactionsbetween pumping elements—crosstalk—to a minimum.

[0043] While only specific embodiments of the present invention havebeen described above, it will occur to a person skilled in the art thatvarious modifications can be made within the scope of the appendedclaims.

What is claimed is:
 1. A piezoelectric activated relay comprising: aliquid metal channel; a first and second set of piezoelectric elements,each of said set of piezoelectric elements forming sidewalls to a firstand second chamber and each of said chambers being connected to saidchannel via a first and second conduit respectively; a first, second andthird contact pad equally separated from each other, each of saidcontact pads having at least a portion within the chamber; and amoveable conductive liquid within the channel, a first portion of theliquid being wetted to the first of said contact pads and a portion ofthe liquid wetted to both the second and third of said contact pads;wherein said chambers and said channel are filled with a fluid andwherein said portion of the liquid wetted to said second and third ofsaid contact pads is moveable toward said portion wetted to the first ofsaid contact pads.
 2. The piezoelectric activated relay of claim 1further comprising a fluid reservoir connected to each of said first andsecond chambers via a first and second through-hole.
 3. Thepiezoelectric activated relay of claim 2 wherein each of said set ofpiezoelectric elements comprises a pair of shear mode piezoelectricelements that can bend toward or away from the cavity between them. 4.The piezoelectric activated relay of claim 3 wherein said fluidreservoir comprises a plurality of compartments wherein each of saidplurality of compartments has compliant walls.
 5. The piezoelectricactivated relay of claim 4 further comprising a relief port connectingsaid plurality of compartments.
 6. The piezoelectric activated relay ofclaim 5 wherein said moveable conductive liquid is moveable by pressuredifferentials created within the first and second fluid chambers causedby activation of at least one set of the piezoelectric elements, saidactivation of said piezoelectric elements causing said piezoelectricelements to deflect in shear causing them to bend.
 7. The piezoelectricactivated relay of claim 5 wherein said moveable conductive liquid ismoveable by pressure differentials created within the first and secondfluid chambers caused by activation of both the first and second set ofthe piezoelectric elements cooperatively with each other.
 8. Thepiezoelectric activated relay of claim 6 wherein said liquid metal ismercury. 9 The piezoelectric activated relay of claim 6 wherein saidliquid metal is an alloy containing gallium.
 10. The piezoelectricactivated relay of claim 7 wherein said liquid metal is mercury.
 11. Thepiezoelectric activated relay of claim 7 wherein said liquid metal is analloy containing gallium.
 12. The piezoelectric activated relay of claim7 further comprising a fill port situated above said fluid reservoir.13. A piezoelectric activated relay comprising: a fluid reservoir layercomprising a fluid reservoir; a piezoelectric layer laminated to saidfluid reservoir layer, said piezoelectric layer comprising a first andsecond set of piezoelectric elements, each of said set of piezoelectricelements forming sidewalls to a first and second chamber and each ofsaid chambers being connected to said channel via a first and secondconduit respectively; a liquid metal channel layer laminated to saidpiezoelectric layer, said channel layer comprising a liquid metalchannel, a first via connecting said channel to the first of saidchambers, a second via connecting said channel to the second of saidchambers, a first, second and third contact pad equally separated fromeach other, each of said contact pads having at least a portion withinthe chamber and a moveable conductive liquid within the channel, a firstportion of the liquid being wetted to the first of said of contact padsand a portion of the liquid wetted to both the second and third of saidcontact pads; wherein said chambers and said channel are filled with afluid and wherein said portion of the liquid wetted to said second andthird of said contact pads is moveable toward said portion wetted to thefirst of said contact pads.
 14. The piezoelectric relay of claim 13,wherein each of said first set of piezoelectric elements comprises atleast two shear mode piezoelectric elements and said second set ofpiezoelectric elements comprises at least two shear mode piezoelectricelements.
 15. The piezoelectric activated relay of claim 14 wherein saidfluid reservoir comprises a single compartment.
 16. The piezoelectricactivated relay of claim 14 wherein said fluid reservoir comprises aplurality of compartments wherein each of said plurality of compartmentshas compliant walls.
 17. The piezoelectric activated relay of claim 16,further comprising at least one relief port connecting each of saidplurality of compartments with adjacent compartments.
 18. Thepiezoelectric activated relay of claim 15 wherein said liquid metal ismercury.
 19. The piezoelectric activated relay of claim 17 wherein saidliquid metal is an alloy containing gallium.
 20. The piezoelectricactivated relay of claim 15 wherein said liquid metal is mercury. 21.The piezoelectric activated relay of claim 15 wherein said liquid metalis an alloy containing gallium.
 22. The piezoelectric activated relay ofclaim 20 wherein said reservoir layer further comprises a fill port. 23.The piezoelectric activated relay of claim 21 wherein said reservoirlayer further comprises a fill port.